We present a unified microscopic approach to four-wave mixing (FWM) in semiconductors on an ultrashort time scale. The theory is valid for resonant excitation in the vicinity of the excitonic resonance and at low densities. The most important many-particle effects, i.e., static and dynamical exciton-exciton interaction as well as biexcitonic effects are incorporated. The internal fields resulting from these interaction processes give rise to pronounced many particle effects in FWM signals. Our results explain the dependence of FWM signals on the polarization geometry, especially if biexcitons contribute. Time-resolved (TR) FWM experiments show that the diffraction of the interaction induced fields dominate the FWM signals completely. This dominance of the interaction induced field at low temperatures is true regardless of density, detuning, or polarization geometry. While spectrally resolved FWM (FWM) shows biexcitonic or bound excitonic contributions under various experimental conditi ons, TR-FWM is always completely delayed, peaking roughly at the dephasing time after both beams passed through

We report on generation of coherent optical phonon oscillations in 150 microm thick bulk GaN. With photon energy far below the band gap, the generation mechanisms of coherent phonon modes of A1(LO), high- and low-frequency E2 are revealed to be the impulsive stimulated Raman scattering. We find that one among the two degenerate E2 modes is selectively detected with a proper choice of probe polarization. Dephasing times range from 1.5 to 70 ps for different modes, and phonon-three-photon absorbed carrier interactions are compared between the A1(LO) and the E2 mode.

Two phase-locked pulses are used to coherently excite excitonic polarizations. It is shown that the second pulse can either be strongly amplified by taking up energy gained from the destruction of the exciton polarization or can be decreased drastically by giving up all its energy to excitons. Both the temporal and the spectral signatures of the transmitted pulse shapes agree well with model calculations.

We report on coherent optical phonon oscillations in wurtzite ZnO. The high- and low-frequency E2 modes are excited by the impulsive stimulated Raman scattering, and detected through the electro-optic effect. The dephasing times are measured to be 1.75 and 29.2 ps for the high- and the low-E2 modes, respectively. The dependency of the phonon amplitude on the spectral width of the femtosecond pulses is shown.

We report on the control of coherent optical phonon oscillations in GaN using the femtosecond pulse shaping technique. The enhancement or the discrimination of the E 2 phonon mode is demonstrated depending on whether or not the spectral separation of the shaped pulse matches the frequency of the phonon. The numerical calculations simulate quite well the lattice motions which are driven by shaped pulses.